Tubes having an internal coating are widely used in industrial and commercial applications. Coatings can serve a variety of purposes, and are used to provide, for example, corrosion protection to tube material under high temperature, pressure, or chemically corrosive conditions that may damage the structural integrity of the tube. Coatings can also improve the wear resistance of tubes subject to harsh conditions and hence lengthen the operating life of such tubes.
In nuclear applications, fuel tube sheaths having interior graphite-based coatings enhance the performance of the fuel. In a CANDU® (CANada Deuterium Uranium) reactor, uranium pellet fuel is loaded into Zircaloy™ sheath tubing that has been pre-coated with graphite. The graphite coating is produced by pre-coating the sheaths with a graphite suspension (i.e. slurry), which is subsequently dried to form the graphite coating.
Various methods of preparing tubes having internal coatings have been described. One commonly employed method is spray coating, wherein a spraying apparatus is advanced along the interior of a tube and a coating material is delivered to the internal surface of the tube by spraying. Tubes can also be coated by applying a coating material inside the tube, positioning the tube in a horizontal position, and rolling the tube to create a relatively uniform coating on the internal surface of the tube. Coatings may be applied, for example, as solutions, suspensions/slurries, and emulsions. Many coatings require drying after application, which can be accomplished either at ambient conditions, or by exposing the wet coating to a gas stream, vacuum, increased temperature, or a combination thereof. Some coatings also require subsequent curing, which often involves elevated temperature, either to achieve the desired properties of the coating or to ensure adherence of the coating material to the tube.
Pigs or plugs adapted to pass inside installed pipelines are known to be useful in re-surfacing the interior of oil, gas, and other fluid conduits. There are different types of pigs known, from simple bullet-shaped pigs to more complex designs including spirally configured devices which turn upon progression through a tube (U.S. Pat. No. 4,425,385), pigs with spaced annular rings (U.S. Pat. No. 4,069,535), and pigs with brushes (U.S. Pat. No. 5,326,400). In use, a fluid is provided to an open end of an elongated pipe, the pig is inserted to the same end of the pipe and the pig is propelled through the pipe. Some pigs can be advanced using a pressurized fluid such as hydraulic fluid or compressed gas introduced to the trailing edge of the pig to cause the pig to be propelled through the pipe and also cause the pig to be sealingly engaged to the inside surface of the pipe. Pigs are also used in the pipeline industry to de-scale and clean the internal surface of operational pipelines to decrease heat or fluid loss and to improve the integrity and lifetime of the pipe.
One tube coating method known as ‘flood coating’ includes filling a tube with a coating fluid which is then allowed to drain from the tube by gravity. Low viscosity coating fluids are used with this process to provide a thin coating layer less than approximately 10 microns thick. In a flood coating method, a low viscosity coating fluid is required to provide a uniform coating surface. Coatings formed by this method often have a gradient as the thickness of the coating increases from the top of the tube to the bottom. To increase the overall coating thickness using flood coating, after the coating layer is dried an additional flood coating can be applied. However, after more than a few coatings, the coating layer reaches a maximum thickness where it cannot be further increased. This is due to the coating fluid starting to dissolve the previously deposited coating layer(s). Furthermore, the multiple application of coating layers to achieve a desired final coating thickness is time-consuming, laborious, and therefore expensive.
Improvement in the application of coatings to internal surfaces of tubes is therefore desirable.